spec: Migrate to SPDX license

Both Fedora and openSUSE now recommends to use SPDX shortname format
for License.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>

corosync-qdevice.spec.in

@@ -16,7 +16,7 @@ Name: corosync-qdevice
 Summary: The Corosync Cluster Engine Qdevice
 Version: @version@
 Release: 1%{?gitver}%{?dist}
-License: BSD
+License: BSD-3-Clause
 URL: https://github.com/corosync/corosync-qdevice
 Source0: https://github.com/corosync/corosync-qdevice/releases/download/v%{version}%{?gittarver}/%{name}-%{version}%{?gittarver}.tar.gz